Net Loss per Share Attributable to Common Stockholders - Computation of Basic and Diluted Net Loss per Share Attributable to Common Stockholders (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Numerator--basic and diluted:
Net loss	$ (20,423)	$ (20,437)	$ (17,062)	$ (15,236)	$ (14,000)	$ (12,829)	$ (14,916)	$ (14,302)	$ (73,158)	$ (56,047)	$ (73,961)
Net loss attributable to common stockholders--basic and diluted	$ (20,423)	$ (20,437)	$ (17,062)	$ (19,142)					$ (77,064)	$ (56,047)	$ (73,961)
Net loss per share—basic and diluted	$ (0.44)	$ (0.46)	$ (0.42)	$ (0.56)	$ (0.44)	$ (0.41)	$ (0.47)	$ (0.53)	$ (1.87)	$ (1.84)	$ (2.92)
Denominator—basic and diluted:
Weighted-average common shares used to compute net loss per share—basic and diluted	46,054,850	44,156,808	40,609,205	34,328,640	31,640,484	31,630,639	31,605,279	27,023,466	41,308,242	30,490,783	25,371,511
Warrant [Member]
Numerator--basic and diluted:
Deemed dividend due to warrant reset									$ (3,906)